Unity3D UI, calculation for position dragging an item?

These days it's incredibly easy to drag UI elements in Unity: Make a few UI items. Add Component -> Event -> Event Trigger. Drop on the script below. Click to add the four obvious triggers. You're done.

However.

I'm totally lost in the relationship between pointer coordinates and UI coordinates (as seen in RectTransform and so on).

In

DragIt

below: how the hell do you move a UI panel correctly under the finger?

Say you have one large panel, with ten UIButton sitting in the panel with

Dragster

on the buttons. What is the relationship between the RectTransform coords and the mouse pointer ...